Methods of measuring a deep vacuum have existed for some time. The Mercury McLeod Gauge, invented in 1874 by Herbert G. McLeod, is a form of manometer that utilizes a column of mercury to indicate pressure. While this type of gauge is still in use today, its relatively large size and fragility preclude it from being practical for use in most industrial, commercial, and portable applications. Consequently, electronic vacuum gauge devices have largely replaced these gauges.
Electronic vacuum gauges utilize vacuum sensors that are generally of the Pirani, thermocouple, or thermistor type. These gauges operate on the principle that a rate of heat transfer by conduction into a surrounding gas is dependent upon gas pressure. The Pirani gauge, invented by Marcello Pirani in 1906, utilizes a platinum wire heated by an electrical current. As the surrounding gas pressure decreases, the temperature of the wire increases due to the reduction in the heat that is being conducted away from the wire and into the surrounding gas. The resistance of the wire increases with respect to the increasing temperature of the wire. Therefore, the measured resistance of the wire is indicative of the gas pressure of the surrounding gas.
The thermocouple type of gauge utilizes a thermocouple thermally connected to a small wire filament to measure the temperature of the filament, which is heated via an electrical current through the filament. An output voltage from the thermocouple is indicative of the filament temperature, which increases as gas pressure decreases.
A thermistor-based gauge operates similarly to the Pirani gauge, but utilizes a temperature sensitive resistor (i.e., a thermistor) rather than a platinum wire. The advantage to this configuration is that thermistors generally have a much higher resistance than the platinum wires used in Pirani gauges. Accordingly, thermistors exhibit a greater resistance change versus temperature change, thereby making resistance and, therefore, temperature measurements simpler and more accurate. There are two types of thermistor-based gauges, each sensing heat by a different method. The first type relies on a heating element that is in contact with the thermistor. The second type uses an electric current to heat the thermistor, thereby directly affecting the thermistor's resistance.
Any of the above techniques may utilize a temperature increase to indirectly measure pressure or, alternatively, may adjust power to maintain a particular temperature (or temperature differential with the surrounding gas). In the latter case, the power required to maintain the device's temperature can be used as an estimate of vacuum pressure, as it is well known in the art that the square of a thermistor's voltage is indicative of pressure.
All of these vacuum sensing techniques are gas-temperature sensitive, where the amount of heat conducted away from the device and into the surrounding gas at any given gas pressure is dependent upon the difference between the temperature of the device and the temperature of the surrounding gas. Therefore, for accuracy across a broad range of ambient (gas) temperatures with these gauges, some form of temperature compensation must be employed. Generally, the sensing device is maintained at a constant differential temperature from the surrounding gas temperature using a secondary temperature-measuring device. Alternatively, the sensing device is maintained at a constant temperature, a secondary temperature-measuring device being used to compute the differential temperature between the vacuum-sensing device and the surrounding gas. The resulting value is used to adjust the vacuum-sensing device's response to changing pressure.
In practice, the response curve of such a vacuum-sensing device is roughly log-linear between the pressures of 1 and 25,000 microns. In this range, conduction of heat to the gas molecules dictates the response curve. When operated at constant temperature, the power dissipated by the device resembles an “S” curve on a log-linear graph. Above approximately the 25,000-microns mark, convective cooling dominates the curve and the response curve rapidly asymptotes to near the atmospheric value. Below approximately the 1-micron mark, thermal conduction through the device's metallic leads and radiative cooling dominate the response curve, thereby yielding yet another asymptote. Therefore, vacuum sensors based upon the thermal conduction of gas are generally acceptable for use only where the measurements are constrained between the two extremes—i.e., 1 and 25,000 microns. In HVAC service, for example, the approximate range of 10 to 10,000 microns is desirable.
Aside from temperature sensitivities, there are other disadvantageous issues with such existing vacuum sensors. First, the power required to maintain the temperature of the sensor at any given pressure not only depends on the ambient temperature, but also depends upon the construction of the sensor, its overall surface area and geometry, the materials used, the presence of any surface contamination, the diameter, length, and conductivity of the lead wire, the size and geometry of the gas cavity, and a number of other unpredictable variables. The sensor, itself, has a specified tolerance based on its manufacture, which means that the resistance of one sensor at any given temperature may be significantly different than that of another at the same temperature, especially in low-cost applications. Therefore, each sensor possesses its own unique response curve with respect to pressure and, as a result, must be individually calibrated against a vacuum reference to achieve any kind of practical accuracy. Because the response curve is only roughly log-linear, a simple two-point calibration is generally not adequate. Instead, many data points need to be calibrated throughout the specified range of the gauge in question, and over a range of temperatures.
Calibrating a vacuum gauge is difficult, time-consuming, and expensive. A high quality vacuum system is required, along with leak-proof gas connections. A standards-traceable master gauge must be incorporated into the system, and the pressure must be repeatedly changed and stabilized for each calibration point. Such a system can be automated to limit the amount of human interaction and decrease calibration time, but such a system still requires constant and repeated maintenance and requires a significant amount of capital resources. In addition, no field technician or end-user of the gauge will typically have this type of maintenance equipment. Therefore, such a gauge requiring calibration must be sent back to the factory for recalibration and, depending on the application, recalibration is frequently needed. Even after proper calibration, a production gauge may not operate to its published specifications in the field. This may be due to the user simply not operating the gauge at the same temperature as when it was calibrated.
Therefore, a need exists for an electronic vacuum gauge that requires no calibration against a vacuum reference while, at the same time, providing high accuracy across a broad range of ambient temperatures.
Many prior-art gauges utilize field-replaceable, per-calibrated sensors so that, in the case of a sensor failure, the sensor may be replaced without the requirement of recalibrating the gauge instrument. This is generally achieved by stamping a calibration code on the exterior of the sensor, which is input into the gauge instrument in some fashion by the operator. This process is an error prone technique and requires the attention of the operator to perform properly. Therefore, a need exists for an electronic vacuum gauge that automatically acquires calibration information from the vacuum sensor without intervention by the operator.
The accuracy of a vacuum-sensing device may change through time, either through component value changes or through gradual contamination of the vacuum-sensing device. There is currently no method, save utilizing a second known-to-be-good gauge, for determining that a vacuum-sensing device, or its associated gauge instrument, is operating within its specified accuracy. Therefore, a need exists for a vacuum gauge instrument that can automatically determine if it is operating within its specified accuracy and a method for automatically determining with a vacuum gauge instrument if the instrument is operating within its specified accuracy.
As a vacuum sensor is in direct contact with the gas being measured, any contaminants in the gas, such as oil, may contaminate the sensor. This will cause inaccurate vacuum measurements, or will cause the vacuum gauge instrument to cease functioning all together. The vacuum gauge sensor may also become faulty for any of a number of reasons, including physical failure. Therefore, a need exists for a vacuum gauge instrument that can automatically determine if a vacuum sensor is contaminated or faulty due to some other cause and a method for automatically determining with a vacuum gauge if a vacuum sensor associated therewith is contaminated or faulty due to some other cause.
Since a vacuum gauge sensor requires significant power to heat the vacuum-sensing device, such vacuum gauges suffer from short battery life, or require the use of mains power for long-term, continuous operation. Therefore, a need exists for a vacuum gauge instrument with a reduced power requirement for the vacuum gauge device to, thereby, increase the overall battery life or to eliminate the need for mains power and an automatic method for reducing the power requirement of a vacuum gauge device to, thereby, increase overall battery life or to eliminate the need for mains power.
Evacuation procedures generally require achieving a minimum predetermined pressure, and holding at least that minimum pressure for a predetermined amount of time. Generally, this is performed by an operator with a clock. This requires the continuous attention of the operator until the evacuation process is complete. Therefore, a need exists for a vacuum gauge device with an automatic method of monitoring an evacuation process and signaling an operator when the process has been successfully completed.
Yet another evacuation procedure may require the system under evacuation to hold a vacuum for a predetermined amount of time with any increases in pressure being indicative of leaks or moisture in the evacuated system. This is generally preformed by an operator watching the gauge for a period of time and manually computing changes in pressure during that time. Therefore, a need exists for a vacuum gauge instrument that can automatically, and instantly, compute and indicate an accurate leak rate, or the rate of pressure increase versus time.